Stem Cells May Be Secret to Regenerating Fingers and Toes

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Mammals can regenerate the very tips of their fingers and toes
after amputation, and now new research shows how stem cells in
the nail play a role in that process.

A study in mice, detailed online today (June 12) in the journal
Nature, reveals the chemical signal that triggers stem cells to
develop into new nail tissue, and also attracts nerves that
promote nail and bone regeneration.

In mice and people, regenerating an amputated finger or toe
involves regrowing the nail. But whether the amputated portion of
the digit can regrow depends on exactly where the amputation
occurs: If the stem cells beneath the nail are amputated along
with the digit, no regrowth occurs, but if the stem cells remain,
regrowth is possible.

To understand why these stem cells are crucial to regeneration,
researchers turned to mice. The scientists conducted toe
amputations in two groups of mice: one group of normal mice, and
one group that was treated with a drug that made them unable to
make the signals for new nail cells to develop.

They found that the signals that guided the stem cells'
development into nail cells were vital to
regenerating amputated digits. By five weeks after
amputation, the normal mice had regenerated their toe and
toenail. But the mice that lacked the nail signal failed to
regrow either their nails or the toe bone itself, because the
stem cells lacked the signals that promote nail-cell development.
When the researchers replenished these signals, the toes
regenerated successfully.

In another experiment, the researchers surgically removed nerves
from the mice toes before amputating them. This significantly
impaired nail-cell regeneration, similar to what happened to the
mice that lacked the signals to produce new nails. Moreover, the
nerve removal decreased the levels of certain proteins that
promote tissue growth.

Together, the results show that nail stem cells are critical for
regrowing a lost digit in mice. If the same turns out to be true
in humans, the findings could lead to better treatments for
amputees.

Other animals, including amphibians, can also regenerate lost
limbs. For example, aquatic
salamanders can regrow complete limbs or even parts of their
heart — a process that involves cells in their immune system. By
studying these phenomena in other animals, it may be possible to
enhance regenerative potential in people, the researchers said.